Why does it say my email was "rejected by the server because it does not allow relaying" when I try to send it?

I'm able to download/get my email just fine but when I try to send an email, I get the message that my email was "rejected by the server because it does not allow relaying." What gives?

If you try to send an email from your iPhone and get an error message that says "A copy has been placed in your outbox. The recipient was rejected by the server because it does not allow relaying," chances are you need to check your outgoing mail server settings. Here's how:

Excellent! The message I received did not reference "relaying" but simply said the email address "was rejected by the server" (very helpful as usual with error messages, no hint what to do next!) But this fixed it so thanks a lot it would have taken me ages to work this out.

At one point in time my iPhone 6 plus was sending emails, but for some reason it stopped. I followed all the steps above except for when I got to number four. My phone wouldn't except the optional password because it said the server doesn't use password authentication. So I left it blank, selected "SSL", and done. Immediately, my phone started sending emails again.